1. Field of the Invention
This invention relates to the field of photonics, and in particular to planar waveguide echelle gratings.
2. Description of Related Art
In wavelength division multiplexing an incoming light beam must be demultiplexed into its component channels. A device suitable for achieving this is a planar waveguide echelle grating. This consists of a slab waveguide providing an input channel and a plurality of output channels and an echelle diffraction grating etched into a wafer which diffracts the incoming light toward the output channels according to wavelength xcex. The echelle grating is typically xe2x80x9cblazedxe2x80x9d, which means that the reflecting facets are tilted at an angle xcex8 relative to the normal to the grating. The blaze angle is chosen so that the light is diffracted predominantly into the higher orders where the resolution is greater. The blaze angle is optimized for a particular wavelength or channel in a particular order, and this is known as the blaze channel.
It is also advantageous if the grating is based on a Rowland circle design. In such a design the grating facets are arranged along an arc having a radius of curvature R. The diffracted rays come to a focus at points on a circle whose diameter D is equal to the radius of curvature R of the arc along which the grating facets are arranged. Geometric analysis shows that this is true provided the size of the grating is small compared to the diameter of the circle.
In such gratings, the facets are either straight or circular. Interference/phase errors along a facet for a given wavelength may lead to focusing of the input on the output channel with aberrations. This becomes a serious problem when the order or size of the grating is larger and the channel count becomes very high. Depending upon the size of the facets, non-perfect focusing may also lead to diffraction in many orders and eventually into high insertion loss at the required order.
The aberrations due to astigmatic effects lead to imperfect focusing of the grating with phase errors resulting in unacceptable cross talk in high resolution gratings.
According to the present invention there is provided a planar waveguide grating device, comprising a slab waveguide defining an input channel and a plurality of output channels; and an echelle grating having a multitude of facets, each of said facets being blazed with respect to a preselected channel position, and each facet having an elliptical curvature so as to be astigmatic with respect to the input channel and said preselected channel position.
The facets are preferably arranged such that the input and output channels lie on a Rowland circle.
In this specification it will be understood that the expression echelle grating is used to cover any xe2x80x9cstaircasexe2x80x9d like grating, including, echelle, echellette, and echelon gratings. The facets are designed such that the input channel and preselected channel lie at the foci of a ellipse following the curvature of the facets. This design leads to substantially aberration free imaging of the input.
The echelle grating is preferably designed to operate in high orders, for example, at least the 20th order, and preferably greater than the 450th order, for example the 471st order.
In another aspect the invention also provides a method of making a planar waveguide grating device, comprising the steps of providing a slab waveguide defining an input channel and a plurality of output channels; and forming an echelle grating having a multitude of facets, each of said facets being blazed with respect to a preselected output channel, and providing each facet with an elliptical curvature so that it is astigmatic with respect to the input channel and said preselected output channel.
The invention also enables the design of an echelle grating with different groups of facets astigmatically blazed with respect to different preselected output channels.